The yeast mating pathway provides an excellent paradigm for negative growth control and differentiation in higher eukaryotes. The goal of the proposed research is to define how an evolutionarily conserved protein kinase, FUS3, exerts critical functions which regulate the cell cycle and signal transduction during mating and vegetative growth. It is proposed that a combination of genetic and biochemical approaches be taken in an effort to determine: A) How FUS3 simultaneously arrests the mitotic cell cycle through inhibition of a G1-cyclins and activates a receptor/G protein- coupled signal transduction pathway; B) The basis of the functional redundancy between FUS3 and KSS1, a homologous putative protein kinase which also promotes signal transduction. To accomplish these goals, the specific aims of the proposal are: 1) To mutagenize FUS3 to identify the protein domains that specify its multiple functions and to make mutants for subsequent aims. 2) To use a FUS3 immune complex kinase assay to test whether physiologically relevant proteins are either phosphorylated by FUS3 in vitro or associated with FUS3 in vivo, and to examine how FUS3 is regulated in vitro and in vivo. 3) To identify proteins which either interact with FUS3 or are its functional homologues by genetic screens.